Dc side commutated chopper and inverter

ABSTRACT

In a chopper or inverter having power thyristors that are switched on a control energization of a load from a DC source, and having commutating capacitors to which current is diverted from the power thyristors when they are to be switched off, recycling means are provided for recharging each commutating capacitor to bring it back to its original potential after each commutation and prepare it for the next commutation. Such recycling means comprises a thyristor (or a diode) and a reactor, in series, shunted across the capacitor. Several expedients are disclosed for preventing excessive charging of the commutating capacitor.

United States Patent [72] inventors David L. Duff Burlington, Ontario;

Shashl B. Dewan, Toronto, Ontario, both of Canada Nov. 13, 1969 Nov. 16,1971 Marathon Electric Research of Canada, Ltd.

Oakville, Ontario, Canada [21 1 Appl. No. [22] Filed [45] Patented [73]Assignee [54] DC SIDE COMMUTATED CHOPPER AND Electronic Design 1 1,Disregard Load Impedance in SCR inverter Design," pp. 68- 72, May 24,1967.

Primary ExaminerWilliam l-l. Beha, Jr. Attorney-Ira Milton JonesINVERTER 2 Chin, 7 Drawing Figs. ABSTRAC'l: in a chopper or inverterhaving power thynstors that are switched on a control energization of aload from a 1.8. CI- C, ource and having commutating apacitors [0 cur-321/45 ER rent is diverted from the power thyristors when they are to be[51] Int. Cl. l-l02m 7/52 switched n recycling means are provided frecharging [50] Field of Search 321/5,45, 8ach commutating capacimr tobring it back to its original 45 C, 45 11 potential after eachcommutation and prepare it for the next commutation. Such recyclingmeans comprises a thyristor (or [$6] References CM a diode) and areactor, in series, shunted across the capacitor.

UNITED STATES PATENTS Several expedients are disclosed for preventingexcessive 3,386,027 5/1968 Kilgore et a1. 321/ 1 1 charging of thecommutating capacitor.

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521552-11 .ELDEWEH BY ATTORNEY DC SIDE COMMUTATED CHOPPER AND INVERTERThis invention relates to apparatus for commutating power thyristors,thyratrons and other unidirectional controlled switching devices inchoppers and inverters by which current is converted to pulsed oralternating current having any selected one of a range of frequencies;and the invention is more particularly concerned with means forefficiently recharging a commutating capacitor in such apparatus.

Inverter apparatus of the type with which this invention is concernedmay be used, for example, to energize a threephase induction motor. Bymeans of such apparatus, the frequency of AC fed to the motor can beadjusted to any selected one of a wide range of values, thus providingfor adjustable variation of the speed of the motor.

Very generally, such an inverter has a pair of DC input terminals and(in the case of a three-phase inverter) three AC output terminals. Eachof the output terminals is connected to both terminals of the DC supplythrough a pair of unidirectional switching elements. The time duringwhich each of the several switching elements is maintained conductivedepends upon the frequency at which the inverter is required to operateand is governed by control apparatus that can be of a known type.

Desirably the switching elements are thyristors (also known as siliconcontrolled rectifiers or. SCRs,) which have an anode, a cathode and agate, and which have the ability normally to block both forward and backcurrent flow between the anode and the cathode. But when a controlsignal is fed between the gate and the cathode of a thyristor, itbecomes forwardly conductive, and it then remains conductive as long ascurrent continues to flow through it in the forward direction, eventhough the control signal may be terminated. To switch off a thyristorthat is connected with an inductive load, the current that has beenflowing through the thyristor must be diverted at least momentarily toanother part of the circuit. This is referred to as commutating" thethyristor. After a brief interruption of forward current flow through athyristor, it will return to its nonconductive state until a signal isagain applied to its gate. The time interval required for switching offa thyristor can be materially decreased if, instead of merelyterminating current flow through it, a reverse current is appliedbetween its anode and cathode.

What has just been said about thyristors is also generally true of otherunidirectional switching devices, such as thyratrons, that can be usedin chopper and inverter circuits to which this invention relates.Although the invention is applicable to circuits employing such otherunidirectional switching devices, it is herein described, by way ofexample, with particular reference to thyristors, inasmuch as they arenow generally preferred for most chopper and inverter applications.

Those skilled in the art will recognize that an inverter of the typewith which this invention is concerned has many principics in commonwith a chopper circuit and that chopper and inverter circuits posecertain common problems with respect to the commutation of powerthyristors.

It is among the general objects of the present invention to providesolutions to these problems.

The usual means for commutating a power thyristor in a power circuitcomprises a commutating capacitor connected in series with a commutatingthyristor, the series-connected capacitor and commutating thyristorbeing shunted across the power thyristor. A reactance is connected inseries with both thyristors to isolate the DC supply from the powerthyristor during the time it is being commutated.

Normally the commutating capacitor has a charge of such polarity thatwhen the commutating thyristor is switched on, current is divertedthrough the commutating thyristor from the power thyristor to thecapacitor, and the capacitor impresses a back voltage across the powerthyristor. The diverted current charges the commutating capacitor to apotential opposite to that which it originally had. Therefore at the endof each commutation the capacitor has the wrong polarity for the nextcommutation.

In one type of prior apparatus the commutating capacitor is connected ina commutating thyristor network whereby the connections of the capacitorterminals with the power thyristor circuit are in effect reversed witheach successive commutation, so that the charge acquired by thecapacitor at each commutation prepares it for the next commutation byvirtue of the effective reversal of its terminal connections. Such anarrangement is described by Bradley et al. in "Adjustable frequencyinvertors and their application to variablespeed drives, Proc. IEE, Vol.III, No. 11, Nov. 1964, at page 1 836 et seq.

In both choppers and inverters the employment of such a polarityreversal network has several disadvantages, among the more obvious ofwhich are the multiplicity of commutating thyristors required for thereversing network and the complexity of thecontrol apparatus needed for.selectively firing the thyristors comprising that network.

An inverter having such a polarity reversal network normally has all ofits power thyristors commutated simultaneously. This system, which isknown as double-DC side commutation, has the disadvantage that it isimpossible for one or more of the power thyristors to continueconducting while any others are being commutated.

Another commutating expedient used with inverters to avoid theall-or-none disadvantage of double-DC side commutation is an arrangementwhereby the power thyristors comprising one half of the inverter bridgeare commutated alternately with those comprising the other half of theinverter bridge. This is known as single-DC side commutation; and whileit has the advantage of requiring fewer commutating thyristors thandouble-DC side commutation, it also makes for an inverter that cannot beused for all applications that might be desired for it, in that it isnot possible to have successive commutations of a particular powerthyristor without an intervening commutation of another power thyristor.Single-DC side commutation has the further disadvantage of posingproblems with respect to high rates of forward voltage rise on thethyristors.

Arrangements are also known for commutating each power thyristorindividually, independently of the other power thyristors in aninverter, but such commutation arrangements involve a multiplicity ofcomponents, and are therefore high in cost.

By contrast with these prior arrangements, it is an object of thepresent invention to provide commutation means for an inverter of thecharacter described by which the inverter is rendered extremelyversatile, in that a particular power thyristor can be commutatedrepeatedly without an intervening commutation of another powerthyristor, although alternate commutation of the thyristors comprisingeach half of the inverter bridge is also possible; and which commutationmeans has the further advantages of being in itself low in cost andbeing capable of cooperating with a relatively simple and inexpensivecontrol device.

More specifically it is an object of the present invention to providesimple and inexpensive recycling means for the commutating capacitor ofa chopper or inverter of the character described whereby, after eachcommutation, the commutating capacitor can be recharged back to itsoriginal polarity, and which recycling means can be controlled byrelatively simple control apparatus and, as applied to an inverter, doesnot interfere with efficient utilization of the energy diverted from thepower thyristors during commutation.

Another object of this invention is to provide a highly efficientinverter of the character described that feeds back to the DC supplyterminals regenerative energy from an inductive load on the inverter.

It is also an object of this invention to provide an inverter having apair of DC input terminals and three AC output terminals, wherein eachof the output terminals is connected with the DC terminals through apair of power thyristors, one for each DC terminal, which inverter iscapable of providing substantially higher frequency alternating currentsthan prior inverters, and with substantially higher efficiency, owing tothe fact that the power thyristors connected with one DC terminal can becommutated successively, without an intervening commutation of thoseconnected with the other DC terminal.

Another object of this invention, particularly achieved in certainembodiments thereof, is to provide efficient but inexpensive means forlimiting to safe values the rate of current rise through commutatingthyristors in apparatus of the character described, to adapt theapparatus for high-power applications.

With these observations and objects in mind, the manner in which theinvention achieves its purpose will be appreciated from the followingdescription and the accompanying drawings. This disclosure is intendedmerely to exemplify the invention. The invention is not limited to theparticular structure or method disclosed, and changes can be madetherein which lie within the scope of the appended claims withoutdeparting from the invention.

The drawings illustrate several complete examples of physicalembodiments of the invention constructed according to the best modes sofar devised for the practical application of the principles thereof, andin which:

FIG. I is a circuit diagram of a chopper which embodies the principlesof this invention; FIG. 2 is a circuit diagram of an inverter whichembodies the invention; and

FIGS. 3 through 7 are fragmentary circuit diagrams illustrating severalmodifications of the commutating means of the inverter apparatus shownin FIG. 2.

Referring now more particularly to the accompanying drawings, FIG. 1illustrates a chopper circuit which embodies the general principles ofthis invention and which can be used to provide for intermittentenergization of a load from a DC source connected with input terminal Pand N. Each period of energization of the load is initiated by applyingto the gate of a powerthyristor TP a gate-drive signal from a suitablecontrol device (not shown) to render the thyristor conductive. Currentthen flows from the positive DC terminal by way of a bus designated(+)BUS, through the power thyristor TP and the load, and back to thenegative DC terminal N by way of a bus designated (-)BUS.

For terminating current flow through the power thyristor TP, and hencethrough the load 5, current must be diverted from the power thyristor,and for the purpose of such commutation there is provided commutatingmeans shunted across the series-connected load 5 and power thyristor,which commutating means comprises a commutating capacitor C and, inseries therewith, a commutating thyristor TC. To commutate the powerthyristor TP, a signal is applied by the control device to the gateelement of commutating thyristor TC. When thyristor TC is conducting, areactor PRI cooperates with the capacitor to isolate the DC supply fromthe power thyristor TP. In this case the reactor PRI comprises theprimary winding of an energy recovery transfonner Tr, the purpose ofwhich is described hereinafter.

Initially, capacitor C has a charge such that the potential at thecathode of the commutating thyristor TC is negative with respect toterminal N by a predetermined voltage value. Therefore, when commutatingthyristor TC is switched on, and the potential at the anode of powerthyristor TP drops to nearly the potential of the cathode of commutatingthyristor TC,'a back voltage is impressed across the power thyristor,turning it ofi.

The current that had been flowing through the power thyristor and theload, and which is now diverted to flow through the commutatingthyristor TC, charges capacitor C and raises the potential at thecathode of the commutating thyristor TC until it is at least equal tothe potential of DC terminal P. (In fact, there is a tendency for thepotential at that cathode to rise to a positive value above that at P,owing to the fact that the reactor PR! is in a resonant circuit withcapacitor C, but this over-shooting is prevented as explainedhereinafier.) When the potential at the cathode of commutating thyristorTC reaches its maximum positive value, and the capacitor tries to forcecurrent back through that thyristor, thyristor TC will of course beswitched off.

It will be observed that the charge on capacitor C is now of theopposite sign from that which it initially carried, which is to say thatthe charge on capacitor is the reverse of that required for the nextcommutation of the power thyristor TI. For reversing the charge on thecapacitor, a recycling means is shunted across the capacitor,comprising, in series, a recycling reactance Rr and a unidirectionallyconductive device. In the chopper circuit of FIG. 1 the unidirectionallyconductive device is a recycling thyristor TR. At a time aftercommutating thyristor TC is switched off, and before the nextcommutation of power thyristor 1? must be performed, a gate signal isapplied to recycling thyristor TR by the control device. With thyristorTR conducting, capacitor C and recycling reactance Rr are connected in aresonant circuit by which the capacitor is recharged back to itsoriginal potential. The capacitor is of course prevented from losingthis charge of its original potential because the recycling thyristor TRis turned off by the back voltage that appears across it as soon as thecapacitor attempts to discharge.

When the power thyristor TP is commutated, there would be a tendency forforward current to continue to flow through it due to the inductance ofthe load 5; but this is prevented by reason of the provision of aso-called free-wheeling diode Da which is shunted across the load andwhich provides an alternate path for such regenerative current around aloop comprising said diode and the load. Note that the diode Da isarranged to prevent flow of current from the positive bus to thenegative one, and therefore it conducts only for an interval beginningwith switching on of the commutating thyristor TC. It serves to clampthe potential of the cathode of power thyristor TI to the potential ofthe negative DC terminal N.

As briefly noted above, during commutation there is a tendency for thecharge on the capacitor to overshoot in the absence of some means forpreventing this from happening. Thus, if the reactance in series withcapacitor C comprised only the primary winding PRI, it and the capacitorwould comprise a resonant circuit having a low resistance, and thereactance would, in efi'ect, try to charge the capacitor to a reversepotential of a value substantially higher than the value of the originalcharge upon it. A known expedient for preventing such overshooting is toconnect a clamping diode across the reactance, through which the excesscurrent flows and in which it is dissipated.

But in this case it is preferred, instead, to inductively couple withthe reactance comprising the primary PRI a secondary winding, designatedSEC, so that the two windings together comprise the energy-recoverytransformer Tr.

The secondary, in series with a diode db is connected across the buses.The diode is so arranged as to block flow of current from the positiveto the negative bus. During commutation, as the charge on the capacitorreaches the desired potential, the current through the primary windingor reactor PRI decreases rapidly, and this decreasing current throughthe primary induces a current in the secondary SEC which flows in thedirection to forwardly bias the diode db and through it flows back tothe input terminals. The turns ratio of the windings PR] and SEC is sochosen that the diode becomes conductive when the capacitor has beencharged to the desired potential. Such current flow through thesecondary brings about a rapid termination of current flow through theprimary PRI and to the condenser, and at the same time effects return ofenergy to the supply circuit that would otherwise be expended inovercharging the capacitor.

FIG. 2 illustrates an inverter which embodies the principles of thisinvention and which can be used for energizing a threephase inductionmotor from a source of DC that is connected with input terminals P andN. A capacitor Cf connected across those terminals serves as a ripplefilter. The motor or other load is connected with three-phase outputterminals A, B and C.

Each of the input terminals P and N is connected through a reactor witha corresponding inverter bus INV(+)BUS and INV()BUS, respectively. Eachreactor comprises the primary PR1 of an energy-recovery transformer Trand Tr respectively.

Each of the output terminals A, B and C is connected with each of theinverter buses through a power thyristor. Thus terminal A is connectedwith INV(+)BUS through thyristor TP-l and with INV()BUS throughthyristor TP-4. Similarly, terminals B and C are respectively connectedwith lNV(+)BUS through thyristors TP-3 and TP-S and with lNV(-)BUSthrough thyristors TP-6 and TP-2. The thyristors TP-l, TP-3 and TP-Sthat are connected with the positive inverter bus are herein referred toas the upper power thyristors, while the remaining power thyristors aredesignated the lower ones.

The upper power thyristors are all commutated at the same time, butindependently of commutation of the lower ones, by commutating meanscomprising a commutating thyristor TC-U in series with a capacitor C,,;while the lower power thyristors are commutated simultaneously with oneanother by a similar commutating means comprising commutating thyristorTC-L in series with a capacitor C Those skilled in the art willrecognize that for the great majority of applications the proper phaserelationship at the output terminals A, B and C will require or permitsuch simultaneous commutation of all of the upper power thyristors orall of the lower ones, but that for utmost versatility it should bepossible both to commutate the upper and lower power thyristorsaltemately and to commutate either the upper ones or the lower onessuccessively. In either such commutation sequence there is an intervalbetween successive commutations of a set of power thyristors duringwhich recycling of its commutating capacitor C or C can take place.

It will be understood that suitable control apparatus (not shown)applies firing signals to the several thyristors at the proper times tocause the load connected with the output terminals to be energized withthree-phase AC of the desired frequency.

In the case of the inverter illustrated in FIG. 2, regenerative energyfrom the load is fed back to the DC input under some load conditions,and under other load conditions it is recirculated back through the loadcircuit as in the free-wheeling arrangement of the FIG. 1 chopper. Thecircuit means for accomplishing this comprises a pair of diodes for eachof the output terminals A, B and C, through which each output terminalis respectively connected, by way of buses DC(+)BUS and DC(-)BUS withthe input terminals. Thus the terminals A, B and C are connected,through respective upper diodes Da-l, Da-3 and Da-S with DC(+)BUS andthereby with the positive input terminal P, and are respectivelyconnected through lower diodes Da-4, Da-6 and Da-Z and DC()BUS with thenegative input terminal N.

Attention is directed to the fact that the commutating means for theupper power thyristors TP-l, TP3 and TP-S, comprising commutatingthyristor TC-U and capacitor C is connected between the positiveinverter bus and the negative DC bus, while the commutating means forthe lower power thyristors, comprising commutating thyristor TC-L andcapacitor C is connected between the negative inverter bus. and thepositive DC bus. A saturable reactor R is shown connected in series witheach commutating thyristor, for a purpose explained hereinafter.

As'with the chopper circuit of FIG. 1, each of the commutatingcapacitors C and C of the FIG. 2 inverter is recharged to its originalpotential after each commutation. The recycling means for commutatingcapacitor C comprises a recycling thyristor TR in series with a reactorRr the series connected thyristor and reactor being shunted across thecapacitor C The recycling means for capacitor C comprises a thyristorTR, and a reactor Rr similarly arranged.

As with the chopper, the energy recovery transformers of the inverterhave secondary windings SEC that prevent charging of the commutatingcapacitors to an excessive potential and which return excess energy tothe DC supply. The secondary of each transformer Tr and Tr, has aconnection to the inverter bus to which its primary is connected, and isalso connected, in series with a diode Db Db with the DC terminal ofopposite polarity. Each of the diodes Db and Db is arranged to preventflow of current from the positive bus to the negative one.

The operation of the inverter apparatus shown in FIG. 2 is generallysimilar to that of the FIG. 1 chopper. Assume that upper powerthyristors TP--l and TP-3 are conducting and are to be commutated, andthat lower power thyristor TP-2 is also conducting and is to remainswitched on. It will also be assumed that capacitor C has been properlycharged for commutation.

To commutate the upper power thyristors, a gate signal is applied tocontrol thyristor TC-U, and current is thus diverted from TP-l and TP-llto capacitor C As with the chopper circuit, the primary of the energyrecovery transformer TR serves as a reactor that supports the DC inputpotential plus the potential of the initial capacitor charge, and thesecondary of the energy recovery transformer cooperates with its primaryto return to the DC input excess energy which would otherwise tend tocharge the capacitor beyond the desired new potential.

Afier control thyristor TC-U has been switched off in consequence of thenew charge on capacitor C that capacitor is charged back to its originalpotential by switching on its recycling thyristor TR to permit currentto flow in the resonant recycling circuit comprising capacitor C andrecycling reactor Rr It will be understood that commutation of the lowerpower thyristors is accomplished in a like manner, by switching oncommutating thyristor TC-L, and that subsequent recycling of commutatingcapacitor C is effected by switching on recycling thyristor TR to placethat capacitor in a resonant circuit with its recycling reactor Rr,

With either the chopper apparatus of FIG. 1 or the inverter of FIG. 2,there is a relatively high rate of current rise through each commutingthyristor at the time of commutation. The rate of this current risedepends upon the current drawn by the load and the distributedcapacitance in the primary of the energy-recovery transformer. Sincepresently available thyristors can sustain only a limited rate ofcurrent rise without overheating, and since destruction of a thyristordue to heating is cumulative, the practical effect of high rate ofcurrent rise through the commutating thyristors is to requirerestrictions to be imposed upon the load that can be powered through theapparatus and/or upon the maximum frequency of commutation. Note thatsuch limitations tend to be imposed for the sake of the commutatingthyristors, rather than by the power thyristors, and hence the apparatusmust in efi'ect be derated to a power level substantially below thatwhich would be feasible for the power thyristors if only they had to beconsidered.

One means of preventing such excessive current rise through the tendsthyristors is to connect each of them in series with a small saturablereactor R,,,,, as shown in FIG. 2. Preferably each saturable reactor hasonly a very few turns (two, for example) so that its distributedcapacitance is so low as to be negligible. The saturable reactorsupports the voltage when the commutating thyristor is first switchedon, permitting only a relatively slow rise in current through thecommutating thyristor; but because of the saturable character of thereactor its presence does not so limit the potential drop at the anodesof the commutated power thyristors as to significantly delay orotherwise interfere with commutation. However, the saturable reactortends to increase the peak potential to which the commuting capacitor ischarged by the diverted current, and in some cases it may be necessaryto prevent overshooting of the commutating capacitors due to presence ofthe saturable reactors. Where the characteristics of the powerthyristors require that the saturable reactor have a relatively highreactance, such overshooting of the commutating capacitors may exceedtolerablelimits, and under those circumstances the excesscapacitor-charging energy due to the saturable reactor can be eitherdissipated, by means of the apparatus illustrated in FIG. 3 or returnedto the DC input by means of the apparatus shown in FIG. 4.

In FIG. 3, a diode DC, in series with a small resistor Re, is connectedacross the saturable reactor R,,,,. The diode is arranged to provide apath for the reactor current when the voltage across the saturablereactor reverses.

The current fed through this path is dissipated in the resistor.

The arrangement illustrated in FIG. 4, while perhaps more expensiveinitially, is also more efficient and may therefore achieve long runeconomic advantages. In the FIG. 4 modification each saturable reactorR,,,, is provided with a secondary winding R which, in series with adiode Dd, is connected across the DC input terminals. The diode Dd isarranged to block current flow from the positive DC input terminal tothe negative one. When voltage across the primary of the saturablereactor reverses, the diode Dd permits the current induced in thesecondary R to flow to the DC input, and excess energy is thus returnedto the input circuit rather than being allowed to overcharge thecapacitor. The amount of energy thus recovered is dependent upon theturns ratio of the windings of the saturable reactor, which should beselected with reference to the turns ratios of the energy-recoverytransformers Tr and Tr, inasmuch as both secondaries influence theamount of energy that will be returned to the input instead of beingpermitted to overcharge the capacitor. It will be understood that thesecondary of the saturable reactor should in any case have asubstantially larger number of winding turns than its primary.

Obviously the expedients just described with reference to FIGS. 3 and 4are applicable to a chopper as well as to an inverter.

With either the chopper of FIG. 1 or the inverter of FIG. 2, thecoupling between the primary and secondary windings of theenergy-recovery transformers should be as close as possible. With idealcoupling, the voltage across the commutating capacitor would neverexceed E-l-E/n, where n is the turns ratio of the energy-recoverytransformer and E is the voltage across the DC input terminals.

Thus overshooting of the commutating capacitor can be minimized byreducing as much as possible the leakage inductance between the primaryand secondary windings of the energy-recovery transformers, as by theemployment of interleaved windings and powdered cores.

However, in many cases it is not economically feasible to achieve theoptimum coupling between energy-recoverytransformer windings, and in anycase the energy-transformer would have to have an extremely hightums-ratio (large number of secondary turns) to prevent overshooting tothe point where voltage across the capacitor was substantially equal toE. With this in mind, the supplemental energy recovery means illustratedin FIGS. 5 and 6 are useful in many situations.

As illustrated in FIG. 5, a diode De can be connected across eachcommutating thyristor TC and TC, arranged to conduct in the directionopposite to forward current flow through the commutating thyristor. Thediode De is normally nonconducting, but it starts to conduct whenpotential across the capacitor is higher than the voltage across the DCinput terminals, feeding energy back to the input and draining off theovercharge on the capacitor. It thus serves both for a sort of clampingand for energy recovery, and it is to be noted that it supplements theaction of the secondary of its associated energy-recovery transformer.

Alternatively, the excess energy stored in the commutating capacitor byits overcharge during commutation can be returned to the input duringrecycling. The means for accomplishing this, which is illustrated inFIG. 6, comprises a secondary winding Rr coupled with each recyclingreactor Rr and Rr A diode Df is connected in series with each secondaryRr and the series-connected secondary and diode are connected across theDC buses, and thus with the DC terminals. The diode Df is of coursearranged to prevent current flow from the positive DC bus to thenegative one. The amount of energy recovered will depend upon the turnsratio of the windings comprising the recycling reactor and itssecondary. With a 1:1 ratio the voltage across the capacitor afterrecycling will have a value approximately equal to the voltage acrossthe DC input terminals.

The circuit illustrated by FIG. 7 is similar to the basic invertercircuits of FIGS. 2 through 6, but it differs from them in that theunidirectionally conductive device of its recycling means comprises, ineach instance, a diode Dr Dr instead of a thyristor. The FIG. 7arrangement has the advantages of being less expensive in itself (adiode costs much less than a thyristor) and of simplifying the controlapparatus associated with the chopper or inverter, inasmuch as noprovision need be made for feeding gating signals to theunidirectionally conductive devices of the recycling means. However, theFIG. 7 arrangement requires substantially larger recycling reactors Rrand Rr L than the other illustrated embodiments, and, specifically, eachof the recycling reactors in the FIG. 7 version should be on the orderof 8 to 10 times as large as the commutating reactors FRI.

The operation of the FIG. 7 circuit can be considered with reference tothe commutation of the upper power thyristors. When the commutatingthyristor TC-U is switched on, commutation proceeds in the usual manner,and until the voltage across the commutating capacitor C has reachedzero the diode Dr remains rearwardly biased and no current flows throughit or the commutating reactor Rr However as the capacitor continues tobe charged to its new polarity, the diode Dr becomes forwardly biasedand an increasing current flows through it and the recycling reactor,building a voltage across that reactor. After the capacitor reaches thepeak of its charge of the new polarity, and the commutating thyristorTC-U is switched off, there is a tendency for current to continue toflow in the recycling reactor; in the direction forwardly with respectto the diode Dr and since such current is flowing in an effectivelyclosed and isolated circuit comprising the recycling reactor and thecapacitor, it recharges the capacitor back to its original polarity.Thereafter, and until the commutating thyristor TC-U is again gated on,the diode Dr and the commutating thyristor maintain this new charge ofthe original polarity upon the capacitor, the diode by reason of itsbeing reverse biased by it and the thyristor by reason of its beingswitched off.

It will be apparent that during recycling the capacitor C,- and therecycling reactor Rr comprise a resonant circuit, and that theunidirectional current flow which takes place during recyclingcorresponds to one-half of an oscillatory cycle for this circuit. Sinceeach of the recycling reactors Rr and Rr in the FIG. 7 embodiment ismuch larger than its associated commutating reactor PRI, the buildup ofcurrent in it is much slower than that in the commutating branch of thecircuit, and therefore the recycling reactor, even though effectively inthe circuit during commutation, does not adversely affect thecommutating cycle.

Owing to the larger recycling reactors, the recycling time is longer inthe FIG. 7 embodiment of the invention than in the other illustratedembodiments thereof, so that the commutating means is not available tobe used again as soon after each commutation as when a thyristor is usedin the recycling means. This disadvantage is more theoretical than real,however, since recycling is completed at about the same time as thecompletion of flow of current in the associated energyrecovery circuitcomprising the secondary winding SEC, and commutation could not takeplace, anyway, as long as energy recovery is proceeding. Thus the use ofa recycling diode instead of a recycling thyristor would bedisadvantageous only where very fast commutation cycles were required,such that energy recovery was not feasible. For other applications therecycling diode will usually be preferred, since its operation isautomatic, requiring no gating signal from the control device associatedwith the inverter or chopper.

From the foregoing description taken with the accompany ing drawings itwill be apparent that this invention provides a solid state chopper orinverter having power thyristors and having very efficient means forreversing the polarity of the commutating capacitors which are used forswitching off the power thyristors.

What is claimed as our invention is:

l. Commutating apparatus for a switching device having anode and cathodeterminals that are connected with a DC source in a series circuit with aload, and a gate to which a signal can be applied to render theswitching device conductive, conductivity being terminable only byterminating forward current flow through the switching device, saidcommutating apparatus being of the type comprising a capacitor and asecond gated switching device connected with one another in a secondseries circuit that is shunted across the first-mentioned seriescircuit, and reactance means connected with the DC source in a circuitcommon to both of said series circuits, the capacitor being normallycharged to one polarity and conduction of the second switching device,when conducting, providing for diversion of current from thefirst-mentioned switching device to the capacitor to charge the same toopposite polarity, said commutating apparatus being characterized by:

A. a saturable reactor having a primary winding connected in said secondseries circuit to limit the rate of rise of current through the secondswitching device upon its being rendered conductive;

B. a secondary winding on the saturable reactor having a larger numberof turns than the primary winding and inductively coupled therewith;

C. a diode; and

D. means connecting the diode in series with the secondary winding andthe DC source in an energy-recovery circuit across the first-mentionedseries circuit, with the diode arranged to conduct in the direction tofeed energy back to the DC source when the current through the saturablereactor is decreasing.

2. Commutating apparatus for a switching device having anode and cathodetenninals that are connected with a DC source in a series circuit with aload, and a gate to which a signal can be applied to render theswitching device conductive, conductivity being terminable only byterminating forward current flow through the switching device, saidcommutating apparatus being of the type comprising a capacitor and asecond gated switching device connected with one another in a secondseries circuit that is shunted across the first-mentioned seriescircuit, and reactance means connected with the DC source in a circuitcommon to both of said series circuits, the capacitor being normallycharged to one polarity and conduction of the second switching device,when conducting, providing for diversion of current from thefirst-mentioned switching device to the capacitor to charge the same toopposite polarity, said commutating apparatus being characterized by:

a diode connected across the second gated switching device and arrangedto conduct in the direction opposite to conduction through the secondgated switching device, to discharge from the capacitor so much of thecharge of said opposite polarity as is substantially in excess of the DCsource potential,

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1. Commutating apparatus for a switching device having anode and cathodeterminals that are connected with a DC source in a series circuit with aload, and a gate to which a signal can be applied to render theswitching device conductive, conductivity being terminable only byterminating forward current flow through the switching device, saidcommutating apparatus being of the type comprising a capacitor and asecond gated switching device connected with one another in a secondseries circuit that is shunted across the first-mentioned seriescircuit, and reactance means connected with the DC source in a circuitcommon to both of said series circuits, the capacitor being normallycharged to one polarity and conduction of the second switching device,when conducting, providing for diversion of current from thefirstmentioned switching device to the capacitor to charge the same toopposite polarity, said commutating apparatus being characterized by: A.a saturable reactor having a primary winding connected in said secondseries circuit to limit the rate of rise of current through the secondswitching device upon its being rendered conductive; B. a secondarywinding on the saturable reactor having a larger number of turns thanthe primary winding and inductively coupled therewith; C. a diode; andD. means connecting the diode in series with the secondary winding andthe DC source in an energy-recovery circuit across the first-mentionedseries circuit, with the diode arranged to conduct in the direction tofeed energy back to the DC source when the current through the saturablereactor is decreasing.
 2. Commutating apparatus for a switching devicehaving anode and cathode terminals that are connected with a DC sourcein a series circuit with a load, and a gate to which a signal can beapplied to render the switching device conductive, conductivity beingterminable only by terminating forward current flow through theswitching device, said commutating apparatus being of the typecomprising a capacitor and a second gated switching device connectedwith one another in a second series circuit that is shunted across thefirst-mentioned series circuit, and reactance means connected with theDC source in a circuit common to both of said series circuits, thecapacitor being normally charged to one polarity and conduction of thesecond switching device, when conducting, providing for diversion ofcurrent from the first-mentioned switching device to the capacitor tocharge the same to opposite polarity, said commutating apparatus beingcharacterized by: a diode connected across the second gated switchingdevice and arranged to conduct in the direction opposite to conductionthrough the second gated switching device, to discharge from thecapacitor so much of the charge of said opposite polarity as issubstantially in excess of the DC source potential.